1. Field of the Invention
The present invention relates to plasma reactor apparatus and processes. More specifically, the present invention relates to grounding a semiconductor substrate pedestal of a plasma reactor apparatus during a portion of a positive voltage power bias oscillation cycle to increase the energy of ion particles of the plasma to increase the feature charging effects regarding a substrate being etched using the plasma reactor.
2. State of the Art
Higher performance, lower cost, increased miniaturization of electronic components, and greater density of integrated circuits are ongoing goals of the computer industry. One commonly used technique to increase the density of integrated circuits involves stacking multiple layers of active and passive components one atop another to allow for multilevel electrical interconnection between devices formed on each of these layers. This multilevel electrical interconnection is generally achieved with a plurality of metal-filled vias (xe2x80x9ccontactsxe2x80x9d) extending through dielectric layers which separate the component layers from one another. These vias are generally formed by etching through each dielectric layer using etching methods known in the industry, such as plasma etching. Plasma etching is also used in the forming of a variety of features for the electronic components of integrated circuits. In addition, vertical capacitors may be formed by etching the features of the wall of the capacitor in the capacitor dielectric and forming the remaining capacitor structure around the etched dielectric. Typically, the capacitance of the capacitor is proportional to the surface area of the wall of the capacitor etched in the dielectric material.
In plasma etching, a glow discharge is used to produce reactive species, such as atoms, radicals, and/or ions, from relatively inert gas molecules in a bulk gas, such as a fluorinated gas, such as CF4,CHF3, C2F6, CH2F2, SF6, or other freons, and mixtures thereof, in combination with a carrier gas, such as Ar, He, Ne, Kr, O2, or mixtures thereof. Essentially, a plasma etching process comprises: 1) reactive species are generated in a plasma from the bulk gas, 2) the reactive species diffuse to a surface of a material being etched, 3) the reactive species are absorbed on the surface of the material being etched, 4) a chemical reaction occurs which results in the formation of a volatile byproduct, 5) the byproduct is desorbed from the surface of the material being etched, and 6) the desorbed byproduct diffuses into the bulk gas.
As illustrated in drawing FIG. 4, an apparatus 200 used in the plasma etching process consists of an etching chamber 202 in electrical communication with a first AC (Alternating Current) power source 204. The etching chamber 202 further includes a pedestal 206 to support a semiconductor substrate 208 and an electrode 212 opposing the pedestal 206. The electrode 212 is in electrical communication with a second AC power source 214. The pedestal 206 has an AC power source 216. The electrode 212 and power source 214 may be an inductively coupled plasma source, a microwave plasma source, or any suitable type plasma source.
In the etching chamber 202, a plasma 222 is initiated and maintained by inductively coupling AC energy from the first AC power source 204 into an atmosphere of gases in the etching chamber 202 and the plasma 222 which comprises mobile, positively and negatively charged particles and reactive species. An electric field develops in a sheath layer 224 around the plasma 222, accelerating charged species (not shown) toward the semiconductor substrate 208 by electrostatic coupling.
To assist with the etching, the potential difference between the plasma 222 and the semiconductor substrate 208 can be modulated by applying an oscillating bias power from the pedestal power bias source 216 to the pedestal 206, as illustrated in drawing FIG. 5A (showing the voltage profiles during such oscillation). During the positive voltage phase 232, the substrate collects electron current from electrons that have enough energy to cross the plasma sheath 124 having a plasma potential 236 (see drawing FIG. 5A). The difference between the instantaneous plasma potential and the surface potential defines the sheath potential voltage drop 238 (FIG. 5B). Since the plasma potential is more positive than the surface potential, this drop has a polarity that retards electron flow. Hence, only electrons with energy larger than this retarding potential are collected by the substrate. During the negative voltage phase 234, positive ions are collected by the substrate. These ions are accelerated by the sheath voltage drop 238 and strike the substrate.
However, it is known that the plasma etch results, including profile modification, can occur if the features are charged enough to modify the trajectories of the ions and electrons that are injected into these features.
Illustrated in drawing FIG. 6 is the phenomena of electrical charging on a semiconductor device in the process of a plasma etch. A material layer 244 to be etched is shown layered over a semiconductor substrate 242. A patterned photoresist layer 246 is provided on the material layer 244 for the etching of a via. During the plasma etching process, the patterned photoresist layer 246 and material layer 244 are bombarded with positively charged ions 248 and negatively charged electrons 252 . This bombardment results in a charge distribution being developed on the patterned photoresist layer 246 and/or the semiconductor substrate 242. This charge distribution is commonly called xe2x80x9cfeature charging.xe2x80x9d
In order for feature charging to occur, the positively charged ions 248 and the negatively charged electrons 252 must become separated from one another. The positively charged ions 248 and negatively charged electrons 252 become separated by virtue of the structures being etched and by the differences in directionality and energy between the positive ions and electrons as they approach the feature being etched. As the structure (in this example a via 254) is formed by etching, the aspect ratio (height to width ratio) becomes greater and greater. During plasma etching, the positively charged ions 248 are accelerated toward the patterned photoresist layer 246 and the material layer 244 in a relatively perpendicular manner, as illustrated in drawing FIG. 7 by the arrows adjacent positively charged ions 248. The negatively charged electrons 252, however, are less affected by the AC power bias source at the semiconductor substrate 242 and, thus, move in a more random isotropic manner, as depicted in drawing FIG. 8 by the arrows adjacent negatively charged electrons 252. This results in an accumulation of a positive charge at a bottom 256 of via 254 because, on average, positively charged ions 248 are more likely to travel vertically towards the substrate 242 than are negatively charged electrons 252. Thus, any structure with a high enough aspect ratio tends to charge more negatively at photoresist layer 246 and an upper portion of the material layer 244 to a distance A (i.e., illustrated with xe2x80x9cxe2x88x92xe2x80x9d indica) and more positively at the via bottom 256 and the sidewalls of the via 254 proximate the via bottom 256 (i.e., illustrated with xe2x80x9c+xe2x80x9d indica).
As shown in drawing FIG. 7, the negatively charged sidewalls of the top of the opening deflects the positively charged ions 248 in trajectories towards the sidewalls. In addition, the positively charged via bottom 256 also decreases the vertical component of the ion velocity and therefore increases the relative effect of initial deflection. The deflection results in ion bombardment of the sidewalls 258 proximate the via bottom 256. This can generate a portion of the etched feature with a re-entrant profile, as shown in drawing FIG. 7. Such a profile can be useful in etching a number of films. For example, a re-entrant profile in a metal film can increase alignment tolerance to shorts to adjacent contacts by shrinking the size of the metal line as it meets the layer below it. In addition, a xe2x80x9cbulgexe2x80x9d can be etched into dielectric films such as borophosphosiliate glass (BPSG) with these ions. In this case, the feature charging causes a pileup of deflected ions at a location in the feature and some widening of the feature occurs.
As shown in drawing FIG. 8 the negatively charged photoresist layer 246 and the upper portion of the material layer 244 deflect the negatively charged electrons 252 away from entering the via 254 or slow the negatively charged electrons 252 as they enter the via 254, both caused by charge repulsion and both of which can change the etch profile. This type of phenomenon is commonly known as xe2x80x9celectron shading.xe2x80x9d
Thus, it can be appreciated that it would be advantageous to develop an apparatus and a process of utilizing a plasma reactor which maximizes or adds a controllable effect of feature charging while using inexpensive, commercially available semiconductor device fabrication components and without requiring complex processing steps.
The present invention relates to an apparatus and method of reorienting electrons generated in a plasma reactor to minimize the electrons"" ability to penetrate a feature and therefore reduce charging inside the feature.
One embodiment of the present invention comprises an etching chamber in electrical communication with a first power source. The etching chamber further includes a pedestal to support a semiconductor substrate and an electrode opposing the pedestal. The electrode is in electrical communication with a second power source. The pedestal is in electrical communication with an AC power source. The etching chamber includes a second electrode in electrical communication with a second AC power source. The pedestal is further in electric communication with a triggerable, high-speed switch. When triggered, the switch closes to short the pedestal to ground. The AC power source is preferably in electrical communication with the switch through a signal line.
As previously discussed, the potential difference between the plasma and the semiconductor substrate can be modulated by applying an oscillating power from the pedestal power source to the semiconductor substrate. During the positive voltage phase, the substrate collects electron current from electrons that have enough energy to cross the plasma sheath. The difference between the instantaneous plasma potential and the surface potential defines the sheath potential drop. Since the plasma potential is more positive than the surface potential, this drop has a polarity that retards electron flow. Hence, only electrons with energy larger than this retarding potential are collected by the substrate. During the negative voltage phase, positive ions are collected by the substrate. These ions are accelerated by the sheath voltage drop and strike the substrate. However, the present invention comprises the shorting of the pedestal, either in a symmetrical manner or nonsymmetrical manner, during the positive voltage phase (i.e., during the time the negatively charged electrons flow to the wafer). This results in an increase in the electric field that retards electron flow to the wafer.
Negatively charged electrons are less affected by the DC bias at the semiconductor substrate than are positively charged ions and, thus, move in a more random manner. However, the shorting of the pedestal, according to the present invention, alters the difference between the potential of the plasma and potential of the semiconductor substrate for a part of the positive voltage phase. In addition, because the surface potential is made more negative relative to the plasma potential, only higher energy electrons can overcome this increased potential barrier and reach the surface. This results in more charging and a bigger difference between the positive voltage at the bottom of the feature and the negative voltage at the top of the feature. This increases the feature charging effects. In other words, the shorting of the pedestal increases the role of feature charging on the etch results.
The triggerable, high-speed switch is preferably controlled by the power output of the AC power source. Thus, when the power output of the AC power source reaches a first predetermined level, a first signal is sent from the AC power source or from a sensor (not shown coupled with the AC power source) to the triggerable, high-speed switch via the signal line. When the first signal is received by the switch, the switch closes to short the pedestal to ground. A second signal is sent from the AC power source or from a sensor (not shown coupled with the AC power source) to open the switch, which discontinues the grounding of the pedestal. The second signal can be sent when power output of the AC power source reaches a second predetermined level, or after a predetermined duration of time passes.
Thus, the present invention is capable of providing a simple and controllable method of effecting the quality and efficiency of plasma etching and is easily implemented on most existing plasma reactors.
Although the examples presented are directed to the formation of an opening with a plasma etching apparatus, it is understood that the present invention may be utilized in a variety of feature-formation and plasma processes.